1. Technical Field
The present invention relates to a method for growing a zinc-oxide-based semiconductor device and a method for manufacturing a semiconductor device, and more particularly to a method for growing a zinc-oxide-based semiconductor layer on a ZnO substrate and a method for manufacturing a zinc-oxide-based semiconductor light emitting device based on an MOCVD method.
2. Description of the Related Art
A zinc oxide (ZnO) is a direct transition type semiconductor having band gap energy of 3.37 eV at a room temperature, and it is expected as a material for a photoelectronic device in a blue or ultraviolet region. In particular, it has physical properties very suitable for a semiconductor light emitting device, i.e., an exciton binding energy of 60 meV and a refractive index n=2.0. Further, the zinc oxide can be employed for a wide range of devices including surface-acoustic wave (SAW) devices, piezoelectric devices, and the like. Moreover, ZnO as a raw material has the advantages that it is inexpensive and is not harmful to the environment and human bodies.
In general, as a crystal growth method for a zinc-oxide-based compound semiconductor device, an MOCVD (Metal Organic Chemical Vapor Deposition) method, an MBE (Molecular Beam Epitaxy) method, or a PLD (Pulsed Laser Deposition) method is utilized. The MBE method is a crystal growth method in an ultrahigh vacuum, and it has problems that an apparatus is expensive and productivity is low. On the other hand, the MOCVD method has advantages that an apparatus is relatively inexpensive, large-area growth and/or simultaneous multiple-wafer growth is possible, a throughput is high, and the method is excellent in mass productivity or a cost.
Meanwhile, in a conventional single-crystal growth technology for a group-III-V-based compound semiconductor, single-crystal growth can be readily performed on a substrate of the same type of single-crystal. However, utilizing the MOCVD method to directly grow a zinc-oxide-based single crystal (which will be also referred to as a ZnO-based single crystal hereinafter) on a ZnO single crystal substrate was difficult.
More specifically, even if the MOCVD method is utilized to grow a ZnO-based single crystal layer on a ZnO single crystal substrate at a high temperature of, e.g., approximately more than 500° C., a crystal layer having granular, whisker-like, rod-like or disc-like crystals or aggregation thereof is apt to be produced. Moreover, even if a single crystal is grown, the crystal layer is formed to have many regions where crystal axes thereof are slightly deviated from each other. As described above, there was a problem that a high-quality ZnO-based single crystal layer having excellent flatness and crystal orientation could not be grown on a ZnO substrate at a high temperature.
On the other hand, a manufacture problem of a ZnO substrate used for crystal growth is pointed out. For example, impurities contained in an ingot (i.e., a bulk single crystal) manufactured by, for example, a hydrothermal method and present on a substrate surface generate defects or dislocations in an epitaxial growth layer, or a mechanical damage introduced in a substrate slicing process and remaining on the substrate surface generates a defect or dislocation in the epitaxial growth layer (e.g., a Japanese Patent Publication No. 4045499 (which will be referred to as Patent Document 1 hereinafter).
To avoid such problems, a method for performing crystal growth by using a buffer layer is carried out (e.g., the Patent Document 1 and a Japanese Patent Application Laid-open No. 2006-73726 (which will be also referred to as Patent Document 2 hereinafter)). More specifically, in the MOCVD method, there is adopted a so-called buffer layer technology. In this technology, a ZnO crystal is grown on a ZnO single crystal substrate at a growth temperature lower than a temperature for growing a ZnO single crystal, e.g., a low temperature less than 500° C. to form a flat and dense amorphous or particulate polycrystal and a heat treatment is performed at a high temperature of approximately 500° C. or above to restore crystallinity or crystalline quality.
Moreover, development of a method for growing a crystal close to an ideal crystal that has less crystal defects and has excellent flatness and crystal orientation is very important to achieve high performance and high reliability of a semiconductor device using a ZnO-based crystal. In particular, to manufacture a semiconductor light emitting device using the ZnO-based crystal, an n-type ZnO-based crystal layer that enables efficient injection of electrons into a light-emission layer, the light-emission layer having a high light-emission efficiency, a p-type ZnO-based crystal layer that enables efficient injection of holes into the light-emission layer are required. To obtain the respective layers of the semiconductor light emitting device, a crystal growth technology that can achieve a ZnO-based crystal having excellent flatness and crystal orientation and a low density of defects (i.e., Zn deficiency, oxygen deficiency, complex defects) or dislocation (screw dislocation, edge dislocation) must be established.